Conventionally, as a photoconductive material (charge generating substance and charge transport substance), an inorganic material selected from Se, CdS, and Zn has been adopted. However, these inorganic materials have problems in photosensitivity (particularly, photosensitivity in a long wavelength area), thermo-stability, toxicity, and others. Therefore, in recent years, an electrophotographic photosensitive element using an organic material as a photoconductive material (charge generating substance and/or charge transport substance) has been earnestly developed because the organic meterial achieves an excellent film-forming property, the absence of the above problems, and a wide range of choices of materials.
The electrophotographic photosensitive elements using an organic material as a photoconductive material include a) a single-layer electrophotographic photosensitive element, in which a charge transport substance and a charge generating substance made of organic materials are dispersed into a binder resin (binding agent), and b) a laminated (divided by function) electrophotographic photosensitive element, in which a charge generating layer containing a charge generating substance made of an organic material and a charge transporting layer containing a charge transport substance are stacked on a conductive support.
An example of the above single-layer electrophotographic photosensitive element is disclosed in Japanese Published Examined Patent No. 10496/1975 (Tokukosho 50-10496, published in 1975) and Japanese Unexamined Patent Publication No. 65961/1991 (Tokukaihei 3-65961), in which a pigment such as a perylene pigment and a phthalocyanine pigment is used as a charge generating substance.
The single-layer electrophotographic photosensitive element can be readily manufactured at low cost with a operational process causing few toxic matters (ozone). Most of the single-layer electrophotographic photosensitive elements are positively charged because of restrictions on a material property.
Further, the laminated electrophotographic photosensitive element has a simple formulated arrangement, high sensitivity, and high stability so as to be widely adopted as an electrophotographic photosensitive element using organic materials as photoconductive materials. Such a laminated electrophotographic photosensitive element is disclosed in, for example, Japanese Published Examined Patent No. 42380/1980 (Tokukosho 55-42380, published in 1980), in which a charge generating layer containing a specified organic compound and a charge transport substance are combined with each other.
Meanwhile, in order to respond to the increasing needs for digitalization and networking, the number of electrophotographic output apparatuses such as a laser beam printer and a digital copying machine has been rapidly increasing. Such an electrophotographic output apparatus has a semiconductor laser, an LED (Light Emitting Diode), and the like as a digital light source.
It is not possible to adopt a conventional analog photosensitive element for the above electrophotographic output apparatus using a digital light source. This is because the conventional analog photosensitive element has its sensitivity in a visible ray area and most of the elements have peak wavelengths at around 550 nm, so that the sensitivity becomes lower when using a digital light source of the semiconductor laser and the like (generally, its peak wavelength is around 670 nm and 780 nm). Namely, in order to develop a photosensitive element for the electrophotographic output apparatus using a digital light source, it is necessary to adopt a charge generating substance being capable of favorably absorbing light in a near-infrared region regardless of whether it is single-layer or laminated one.
Typical photoconductive charge generating pigments, that exert high sensitivity to light of the near-infrared region, include a metal-containing phthalocyanine pigment such as titanylphthalocyanine, vanadylphthalocyanine, aluminum chlorophthalocyanine, indiumchlorophthalocyanine, galliumhydroxyphthalocyanine, and galliumchlorophthalocyanine; a metal-free phthalocyanine pigment having no metallic atom at the center; and other pigments. Furthermore, although a pigment such as a pyrroles pigment and trisazos pigment is a relatively peculiar material, they can be adopted as photoconductive charge generating pigment.
According to a report, the above-mentioned organic pigments respectively have several to several tens types of specific crystals, and each crystal type has a different degree of sensitivity to light. For example, the metal-containing phthalocyanine pigment such as titanylphthalocyanine shows higher sensitivity for a specific crystal type as compared with the metal-free phthalocyanine pigment. However, the metal-containing phthalocyanine pigment of this type is manufactured in a complicated process because of the need for an industrially complicated operation for conversing crystals, resulting in a large increase in the manufacturing cost.
Moreover, a pigment such as a pyrroles pigment and trisazos pigment is so special that such an material is more expensive than the typical metal-free phthalocyanine pigment and the disazo pigment that has been widely used for an analog photosensitive element.
For this reason, a large number of electrophotographic sensitive elements using the metal-free phthalocyanine pigment, which is relatively inexpensive as a charge generating substance (see Japanese Unexamined Patent Publication No. 150255/1987 (Tokukaisho 62-150255, published on Jul. 4, 1987), Japanese Unexamined Patent Publication No. 226156/1987 (Tokukaisho 62-226156, published on Oct. 5, 1987), Japanese Unexamined Patent Publication No. 128890/1995 (Tokukaihei 7-128890, published on May 19, 1995), and Japanese Unexamined Patent Publication No. 278649/1996 (Tokukaihei 8-278649, published on Oct. 22, 1996)).
However, with the electrophotographic photosensitive element using the metal-free phthalocyanine pigment, the material cost can be reduced; however, it is difficult to sufficiently obtain a property (property for an actual use of electrophotography) of the photosensitive element.
To be specific, for instance, the electrophotographic photosensitive element has less stable sensitivity. Further, when the sensitivity is repeatedly used, problems such as reduction in charge potential and an increase in remained potential cause an unstable electrostatic property. As a result, in the electrophotographic output apparatus, a defect of image concentration and an increased fog density, so that it is difficult to achieve high reliability. The above-mentioned problems occur in the laminated photosensitive element being more superior than single-layer one in its property as well as in the single-layer photosensitive element using the metal-free phthalocyanine pigment as a charge product.